This invention relates to casting of metal ingots and, more particularly, to a new and improved method and apparatus for continuous casting of ingots having uniform grain structure and to the ingots produced thereby.
For certain applications, such as components of aircraft engines and the like, it is important to obtain an ingot of metal alloy material which has a substantially uniform grain structure. Efforts have been made in the past to produce uniform ingots by various techniques. In the patents to Hunt, Nos. 4,583,580 and 4,681,787, for example, a continuous casting method is described in which the alloy to be continuously cast is heated in a cold hearth electron beam furnace and the temperature of the alloy and the hearth is controlled so as to maintain a solids content of about 15% to 40%. The molten mixture poured from the hearth to the casting mold thus has a high content of solid material, and it is poured into the mold with a substantial vertical velocity so as to distribute the liquid-solid mixture throughout the pool of molten material at the top of the mold. As a result, the mixture in the mold has a substantially thixotropic region with a solids content of at least 50%.
To prevent hot tears in the side walls of an ingot being cast continuously, the Lowe Patent No. 4,641,704 discloses vertical pouring of successive equal-volume quantities of molten material from a launder disposed above the top of the mold into the central portion of the mold at spaced time intervals with intermittent cooling and lowering of the ingot in the mold.
Another approach for providing uniform-grain ingots described, for example, in Hunt Patents Nos. 4,558,729 and 4,690,875, utilizes a rotating mold structure into which molten drops of the ingot material fall and solidify. The mold is maintained at a temperature which is below the solidus temperature of the ingot material, but above a temperature at which metallurgical bonding of the successive molten drops can occur, thereby producing an ingot without altering the grain size and distribution of the metal drops.
Such techniques are not only complicated and difficult to execute, but also place limitations on the size and shape and properties of the resulting ingot.